Like the other games, there are numerous characters within the game who will gladly trade their Pokémon for one of yours. Pokémon Black 2 & White 2 continue with this and have the most in-game trades of any Pokémon game and some include Pokémon with their Hidden Ability

Route 4 - Petilil/Cottonee

Location: Route 4

Method:
When you reach Route 4, you will encounter a trainer looking for one of the native Pokémon of the game. In exchange, she will give you the counterpart Pokémon that is not in the game.

Method:
To get these trades is rather convuluted. You first need to find the Dropped Item in Nimbasa City. From there, you will be called when you enter random areas by the owner of the item. After the 10th call, the person will call you and tell you to meet in Nimbasa to return their Item. When you do, they will be added to your Xtransceiver. Call them to total 30 times, but you can only call from places which fulfill the conditions. On the 30th call they'll tell you they want to meet at Nimbasa City where they'll ride the Ferris Wheel with you. Call them to total 50 times, in the 40th call they'll be 'SOUND ONLY', in the 50th call they'll be wearing their work clothes, and cut the line in a second or so. Call them a 51st time. wait a day and then call them again and they'll meet you in Nimbasa City and offer to trade, willing to have any Pokémon. This can be repeated each day to go through all of their Pokémon. The Pokémon you receive varies based on your gender

The areas are as follows:

Behind the Pokemon Centre in Nimbasa City

Near the Officer in Route 16

Behind the fence on the bottom of Route 5

Somewhere near the slope to Clay Tunnel in Driftveil City

Across the first bridge and near the ledges of Route 6

Around the stairs near the Breeder in Route 7

On the Airstrip of Mistralton City

Route 9 near the Hidden Grotto

Lentimas Town near the stairs.

Undella Town between 2 rocks

On the beach in Route 13 near the start from Undella Town which is behind the ledge